Pick and place machines are generally used to manufacture electronic circuit boards. A blank printed circuit board is usually supplied to the pick and place machine, which then picks electronic components from component feeders, and places such components upon the board. The components are held upon the board temporarily by solder paste, or adhesive, until a subsequent step in which the solder paste is melted or the adhesive is fully cured.
Pick and place machine operation is challenging. Since machine speed corresponds with throughput, the faster the pick and place machine runs, the less costly the manufactured board will be. Additionally, placement accuracy is extremely important. Many electrical components, such as chip capacitors and chip resistors are relatively small and must be accurately placed on equally small placement locations. Other components, while larger, have a significant number of leads or conductors that are spaced from one another at a relatively fine pitch. Such components must also be accurately placed to ensure that each lead is placed upon the proper pad. Thus, not only must the machine operate extremely fast, but it must also place components extremely accurately.
In order to enhance the quality of board manufacture, fully or partially populated boards are generally inspected after the placement operation(s), both before and after solder reflow, to identify components that are improperly placed or missing or any of a variety of errors that may occur. Automatic systems that perform such operation(s) are highly useful because they help identify component placement problems prior to solder reflow. This allows substantially easier rework and/or the identification of defective boards after reflow that are candidates for rework. One example of such a system is sold under the trade designation Model KS Flex available from CyberOptics Corporation of Golden Valley, Minn. This system can be used to identify such problems as alignment and rotation errors; missing and flipped components; billboards; tombstones; component defects; incorrect polarity; and wrong components.
Identification of errors pre-reflow provides a number of advantages. Rework is easier; closed-loop manufacturing control is facilitated; and less work in-process exists between error generation and remedy. While such systems provide highly useful inspection, they do consume plant floor-space as well as programming time and maintenance efforts.
One relatively recent attempt to provide the benefits of after-placement inspection located within and pick a place machine itself is disclosed in U.S. Pat. No. 6,317,972 to Asai et al. That reference reports a method for mounting electric components where an image of a mounting location is obtained prior to component placement, and compared with an image of the mounting location after component placement to inspect the placement operation at the component level.
While the disclosure of Asai et al. marks one attempt to employ in-machine component level inspection, there remains much work to be done. For example, the disclosure of Asai et al. teaches acquiring two images, before and after the placement of the component at exactly the same time in the placement cycle. While this approach is useful for determining the absence or presence of a component after placement, there are several practical problems in the approach. First, image acquisition is very important and requires precise timing control to align the images to proper time slots. Secondly, even if predetermined triggers of the image acquisitions result in aligned images, there are sometimes requirements that the trigger be adjusted to optimize the acquisition timing to the particular part or placement location on the board. Additionally, depending on the measurement required, the relative timing of the image acquisition may require adjusting during the normal cycling of the machine.
To increase the viability of component level placement inspection in a pick and place machine, it would be advantageous to improve aspects of the image acquisition timing, allowing images to be acquired at predetermined times relative to the placement cycle and to allow the image acquisition timing to be changed on-the-fly using software commands. This would allow the optimal timing to be used for each of the placements.
In addition to the timing aspects of the pick and place machine, the actual placement cycle of a component may not be regular. If a component is not properly recognized by the component alignment camera or if the component is not present on the nozzle during the operation of a turret style pick and place machine, the placement cycle is skipped and the component is repicked at later time in the placement cycle. When this operation occurs, the placement inspection device should determine if the lack of component after placement is due to a planned non placement or an actual misplaced component.